The invention relates to a carrier detector in a modem provided with an echo canceler, the carrier detector comprising a first detection circuit which processes a signal representative of the level of the output signal of the echo canceler.
Generally, this carrier detector is included in the receiver of a modem utilizing modulation of a carrier for the data transmission. During certain procedures this detector is used, for example, to indicate whether the modem at the other end of the line transmits or does not transmit a carrier so that the data reproduced by the receiver are only validated when a carrier is transmitted.
A known carrier detection circuit consists solely of a threshold circuit which processes a signal representative of the level of the received carrier to indicate whether this received carrier is present or not.
However, a special carrier detection problem is encountered in a modem which is provided with an adaptive echo canceler for automatically canceling the echo signals which may reach the receiver. An echo canceler for data transmission is disclosed in, for example, applicant's French Patent Application No. 2,377,734. By means of modems comprising an echo canceler, data can be simultaneously transmitted in both directions over one single transmission line. It is clear that in a modem comprising an echo canceler the carrier detector must be arranged behind the output of the echo canceler in order to detect only the carrier coming from the other end of the line, for, during simultaneous transmission in both directions, there remains in practice at the output of the echo canceler a residual echo signal whose level E.sub.r is coupled to the level S of the carrier received from the other end in accordance with the formula: ##EQU1##
In this formula, which is derived in an article by Mueller in IEEE Transactions, vol. COM-24, no 9, September 1976, pages 956-962, N is the number of filter coefficients of the echo canceler and .alpha. is a constant coefficient small with respect to 1, which coefficient determines the magnitude of the increment by which these filter coefficients are modifed.
In practice .alpha.N is small with respect to 2 and the above-mentioned formula may be replaced by the approximation; ##EQU2##
In order to reduce the ratio E.sub.r /S, the coefficient .alpha. can be reduced but this possibility is limited by the resultant increase in the convergence time of the echo canceler. In practice a coefficient .alpha. is chosen resulting on the one hand in a reasonable convergence time and on the other hand in such a ratio E.sub.r /S that the receiver can easily distinguish the data transmitted by the carrier having the level S in the presence of the residual echo signal having the level E.sub.r. Finally, for a given modem the residual echo signal has a level E.sub.r which has a given deviation of -K dB, for example -20 dB, with respect to the level S of the carrier.
As regards the considered carrier detector, it should be noted that after an interruption of the carrier the signal at the output of the echo canceler is not immediately canceled because of the convergence time of the echo canceler. A time interval of several seconds may elapse before this cancelation is achieved. The signal produced at the output of the echo canceler after the interruption of the carrier has at first the level E.sub.r of the residual echo signal which deviates by -K dB with respect to the level S of the carrier before the interruption.
On the other hand, the transmission medium of the facility connecting the two modems has an attenuation which is not known, particularly when the connection is effected through the switched public telephone network; this attenuation may vary between 0 dB and a maximum value of A dB, for example equal to 48 dB. From this it follows that, relative to the level of the transmitted carrier taken as a reference, the level S of the received carrier varies in the range between 0 dB and -A dB. Relative to the same reference level, the level E.sub.r of the residual echo signal after interruption of the carrier varies in the range between -K dB and -(A+K) dB. It is clear that the level of the residual echo signal after an interruption of a high level carrier may be higher than the level of a low level carrier and that the prior art detection circuit processing a signal representative of the level of the output signal of the echo canceler runs the risk of not detecting a carrier interruption.
In order to achieve, that the known detection circuit detects a carrier interruption, the level E.sub.r of the residual echo signal which has a value (S-K)dB must evidently be lower than the lowest carrier level, that is -A dB. This condition may be written as EQU S&lt;(-A+K) dB.
In other words, the known carrier detection circuit can only detect a carrier interruption in the range of the low carrier levels located between the minimum level -A dB and the level (-A+K) dB, for example, between -48 dB and -28 dB in the example where A=48 dB and -K=-20 dB. In the range of higher carrier levels up to the maximum level of 0 dB, the known carrier detection circuit cannot detect the carrier interruption.